Follow up idol (3)
I believe the answer is:
god
'idol' is the definition.
(I know that idol can be written as god)
'follow up' is the wordplay.
'follow' becomes 'dog' (dog can mean to follow or torment).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'dog' reversed gives 'GOD'.
(Other definitions for god that I've seen before include "Pan perhaps" , "Being upstairs" , "Worshipped figure" , "Superhuman being" , "Creator of the universe" .)